1. Field of the Invention
The present invention relates to a heating apparatus for a vehicle, in which a heat-generating unit using a shearing force, for increasing a temperature of cooling water for cooling a water-cooled engine is disposed in a cooling water circuit.
2. Description of Related Art
Conventionally, as a heating apparatus for a vehicle, a hot water type heating apparatus for heating a passenger compartment has been generally known. In the hot water type heating apparatus, cooling water for cooling a water-cooled engine is supplied to a heater core disposed in a duct, and air heated while passing through the heater core is blown into the passenger compartment by a blower to heat the passenger compartment.
However, in a case of a vehicle in which an amount of generating heat of the engine is too small to heat the cooling water with the engine sufficiently, such as a vehicle having a diesel engine or a lean burn engine, because a temperature of the cooling water in the cooling water circuit cannot be maintained at a predetermined temperature (for example, 80.degree. C.), there occurs a problem in that a heating capacity for the passenger compartment is insufficient.
To overcome such a problem, as disclosed in JP-A-2-246823, JP-A-3-57877, and JP-A-2-254010, the following heating apparatus for a vehicle has been conventionally proposed. In the heating apparatus for a vehicle, a heat-generating unit using a shearing force, for heating cooling water supplied to a heater core, is disposed in a cooling water circuit, the heat-generating unit is operated when the cooling water temperature is equal to or lower than a set cooling water temperature to improve the heating capacity for the passenger compartment.
The heat-generating unit transmits a rotational driving force of the engine to a shaft through a belt transmitting mechanism and the electromagnetic clutch. In a case of the unit, there are formed a heat-generating chamber and a cooling water passage at an outer periphery of the heat-generating chamber. A rotor which rotates integrally with the shaft is disposed in the heat-generating chamber, and a shearing force generated by a rotation of the rotor is applied to viscous fluid such as silicon oil sealed in the heat-generating chamber to generate heat. The cooling water is heated by the generated heat.
However, according to the above-described conventional heating apparatus for a vehicle, since the rotational driving force (driving torque) acting on the shaft of the heat-generating unit is converted into an auxiliary heating source, a large load due to the driving torque (driving load) is applied to the engine, thereby causing a problem in that fuel consumption rate of the engine lowers and running cost of the vehicle rises.
To prevent the heat-generating unit from being operated excessively, the set cooling water temperature may be lowered so that deterioration of the fuel consumption rate can be suppressed. However, in this case, because the temperature of the cooling water cannot be maintained at a predetermined temperature, a radiating amount of the heater core disposed in the duct is lowered, thereby causing a problem in that a sufficient heating capacity cannot be obtained.